For a considerable period of time and especially since about 1955 there have been proposals for use and the actual industrial use of insoluble, dimensionally stable anodes in electrochemical processes involving, among others, and anodic evolution of oxygen and chlorine. The term "dimensionally stable" refers to insoluble valve or film-forming metal substrate anodes which do not suffer shape modification during use as do other insoluble anodes such as, for example, graphite or Pb-based anodes. By valve metal one refers to metals, typically characterized by titanium, which permit the flow of current when used under cathodic conditions but do not permit the flow of current when used under anodic conditions due to the rapid oxidation of the metal which results in an adherent, substantially continuous non-conductive oxidic film on the metal.
Insoluble, dimensionally stable electrodes (IDS electrodes), such as disclosed in U.S. Pat. Nos. 3,103,484; 3,547,600; 3,663,414; 3,677,815; 3,773,555; 3,950,240; 3,956,083; 4,028,215; 4,070,504; 4,052,271 and variants thereof have found widespread industrial use. These IDS electrodes typically comprise a metal substrate having on and adhered to the surface thereof either some platinum-group metal or combination of platinum-group metals or some oxide or oxidic combination having reasonable electronic conductivity. The material adhering to or coating the substrate surface is insoluble in the anolyte environment in which it is to be used, and advantageously has a low overpotential for oxygen evolution. The material of the coatings on the valve metal substrate can be costly. However, the coatings are usually very thin and thus the precious metal is used in a cost-effective manner.
What is less apparent from a cost standpoint is the cost of the valve metal substrate. Commercial use of such IDS anodes generally employs relatively large sheets of valve metal or so-called expanded metal mesh of the valve metal. These substrate forms are quite expensive. In addition, the practical requirements of good current distribution over the anode make it imperative to have a substrate of low electrical resistivity. Often this low electrical resistivity requirement necessitates the welding of current carrying bus-bars to the substrate, thus adding to the complexity of anode manufacture and increasing substrate cost. Alternatively, an anode having a larger cross-sectional area can be used to give good current distribution from the top of the anode to its bottom, but by this technique s substantially greater weight of valve metal is required and therefore substrate cost increases.
While titanium prices have varied considerably in the past depending principally upon the demand for the metal in its particular forms, it is always true that the cheapest form of titanium available is sponge titanium and that sponge titanium powder, being a byproduct of sponge, is generally cheaper still.
It is the object of the present invention to provide a means and method whereby one can produce satisfactory electrode substrate sheet using titanium sponge powder at a cost substantially less than the cost of solid titanium in the form of sheet, rod or expanded mesh.
It is a further object of this invention to provide a process for manufacture of a porous substrate that can have superior electrochemical characteristics because of its relatively large surface area relative to solid titanium, the result of which is to reduce the local current density at the substrate surface or electrochemically active coated surface and thus give longer life of the substrate under service as an electrode.